The drive toward higher levels of integration in VLSI circuits has led to a need for submicron-scale exposure technology in the photolithographic systems used to form integrated circuit patterns on wafers. Light sources of increasingly shorter wavelength are being employed in exposure systems to carry out patterning to smaller linewidths. The i-line (wavelength, 365 nm), once the light source of choice in lithography steppers, has been largely supplanted by the KrF excimer laser (248 nm), and today ArF excimer lasers (193 nm) are starting to see industrial use.
In the liquid crystal display (LCD) field, large-sized synthetic quartz glass substrates are currently employed as photomasks while they require image writing feature sizes of the order of several generations ago, as referred to in the semiconductor technology. From now on, the image writing feature size will decline in a direction following the technical transitions in the semiconductor technology. It is expected that large-sized photomask substrates for LCD will be required to have a quality approximate to that of the semiconductor photomask substrates.
Synthetic quartz glass is generally prepared by several processes so as to avoid contamination of metal impurities which cause UV absorption. One exemplary process, which is commonly known as “direct process,” is by introducing a vapor of a high purity silicon compound such as silicon tetrachloride directly into an oxyhydrogen flame, subjecting the compound to flame hydrolysis to form fine particles of silica, depositing the silica particles directly on a rotating heat resistant substrate of quartz glass or the like, and concurrently melting and vitrifying the particles to form synthetic quartz glass ingot. Another process, which is commonly known as “soot process,” is by depositing and sintering soot on a heat resistant substrate and then vitrifying the deposit into transparent glass in an electric furnace or the like. In either way, transparent synthetic quartz glass members are produced.
Synthetic quartz glass products thus prepared have satisfactory light transmission even in the short wavelength region down to about 190 nm. They are thus widely employed as transmissive materials for UV radiation including i-line, excimer lasers such as KrF (248 nm), XeCl (308 nm), XeBr (282 nm), XeF (351, 353 nm) and ArF (193 nm), the 4-fold harmonic of YAG (250 nm), and the like.
From the synthetic quartz glass ingots prepared by the above-mentioned processes, synthetic quartz glass substrates are produced, for example, by hot shaping the ingot into a quartz block of the desired shape, annealing for removing thermal strain, slicing the block into plates, and polishing. The resulting synthetic quartz glass substrates are ready for use as semiconductor photomask substrates.
Recently, synthetic quartz glass ingots by the direct process are often used in the production of large-sized synthetic quartz glass substrates for LC photomasks which are used in the LCD manufacture process.
However, large-sized photomask substrates for LC use are increasing in size with the advancing generation of LCD. The current photomask substrates have dimensions of 1,220 mm×1,400 mm at maximum. In preparing stocks for such large-sized substrates, the soot process may be used which includes vapor-phase axial deposition (VAD) and outside vapor deposition (OVD) processes. Of these, the VAD process can produce only synthetic quartz glass ingots of light weight and is difficult to produce large-sized substrates. The OVD process can produce synthetic quartz glass tubes of heavy weight, which can be processed into plates by the technique disclosed in U.S. Pat. No. 6,319,634. However, the overall process has the drawbacks that a number of steps are involved and fine bubbles resulting from unmelted soot residues can be left in the glass.
Thus, in the prior art, synthetic quartz glass ingots are formed by the direct process in a thick and long form so as to provide a weight gain, thus obtaining stocks for large-sized substrates. However, when the diameter of an ingot is increased above a certain level, it becomes difficult to maintain the shape of the ingot growing face unchanged. For example, if an ingot has a diameter as large as 200 mm, the shape of the growing face is distorted and becomes irregular, interfering with continuous growth. Then the supply of silicon compound feedstock is interrupted at predetermined intervals, the irregular surface resulting from distortion of the growing face is corrected solely by an oxyhydrogen flame. See U.S. Pat. No. 7,232,778 or EP-A 1329429 corresponding to JP-A 2003-176142. In this case, striae are formed in a plane perpendicular to the ingot growth direction. Although the striae are observable in a perpendicular direction to the synthetic quartz glass ingot growing direction, the ingot is shaped in a direction parallel to the ingot growth direction and the surface of the shaped member serving as a substrate has the same direction so that no striae are inspected. However, as the substrate size becomes larger, striae are sometimes observed near the periphery of substrates. It is believed that since the spacing between striae is narrow at a curved peripheral portion of a growing synthetic quartz glass ingot, such closely packed striae become visible. It is forecast that these striae will give rise to optical problems when a finer feature size becomes necessary in the LC field.